line of scrimmage

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Either of two imaginary lines extending across the field parallel to the goal line at the ends of the ball as it rests prior to being snapped and at which each team lines up for a new play.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun American football Either of two imaginary lines across the football field touching one of the two apices of the football which point toward either the defensive side or the offensive side before the center is allowed to touch the ball before each play.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun line parallel to the goal lines where football linesmen line up at the start of each play in American football